1. Field of the Invention
The present invention relates generally to a light assembly, and more particularly to a backlight assembly for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (LCD) is a passive device that does not emit light. A liquid crystal display (LCD) device displays images by reflecting natural light that is incident on a liquid crystal (LC) panel of the LCD device and/or by using a backlight assembly that is incorporated into the LCD device.
The backlight assembly is generally classified into an edge-type and a direct-type, according to the position of the light source. In the edge-type backlight assembly, the light source is positioned near the edges of the LC panel. Thus, the edge-type backlight assembly needs a light guiding plate to guide the light to the LC panel disposed over the light guiding plate. In contrast, the direct-type backlight assembly does not need a light guiding plate because the light source is disposed under the LC panel and the light from the light source is directly incident on the LC panel. The edge-type backlight assembly is further classified into a flat-type and wedge-type according to the shape of the light guiding plate.
FIG. 1A is a cross sectional view of a conventional flat-type backlight assembly, and FIG. 1B is a cross sectional view of the conventional wedge-type backlight assembly.
As shown in FIG. 1A, a first and second lamp reflectors 12a and 12b are disposed near two opposing edges of the flat-type light guiding plate 10 for preventing light leakage. A first and second lamp units 14a and 14b are disposed on an inner surface of the first and second reflectors 12a and 12b, respectively. The light guiding plate 10 directs the light generated from the first and second lamp units 14a and 14b toward optical sheets 16. A reflective sheet 18 is disposed on a surface of the light guiding plate 10 that is opposite the surface that is closest to the optical sheets 16. The reflective sheet 18 reduces light leakage by reflecting any leaked light back toward the optical sheets 16. “Light leakage,” as used herein, is light traveling in directions that are not conducive to enhancing the luminance of the display device, e.g. away from the direction of the LC panel.
As shown in FIG. 1B, in the wedge-type backlight assembly, a lamp reflector 22 is disposed near one end of a light guiding plate 20 to prevent light leakage. A lamp unit 24 is disposed on an inner surface of the lamp reflector 22. The light guiding plate 20 guides the light from the lamp unit 24 to optical sheets 26. A reflective sheet 28 is disposed on a surface of the light guiding plate 20 that is opposite the surface on which the optical sheets 26 are disposed. The reflective sheet 28 reduces light leakage by reflecting light toward the optical sheets 26.
A problem with these backlight assemblies is that the reflective sheet expands with temperature. The degree of expansion varies as a function of distance from the lamp, and when different parts of the reflective sheet expand by different amounts, the reflective sheet loses its flatness. As shown in FIG. 1A and 1B, the light guiding plate 10/20 presses down on the reflective sheet 18/28 and is positioned to receive light that is reflected by the reflective sheet 18/28. When the reflective sheet loses its flatness, however, not all the light is reflected in the intended direction. Thus, the uneven expansion of the reflective sheet 18/28 results in degradation of display quality.
A method is needed to reduce the negative effects caused by an uneven expansion of the reflective sheet, thereby preventing degradation of display quality with increased temperature.